Observation of the decay $B^0\to J/\ddot{I}ˆ\ddot{I}‰$ at Belle II

Abstract

We measure the branching fraction of the decay $B^0\to J/\ddot{I}ˆ\ddot{I}‰$ using data collected with the Belle II detector at the SuperKEKB collider. The data contain $(387\pm 6)\times 10^6$ $B\overline{B}$ meson pairs produced in energy-asymmetric $e^{+}{e}^{-}$ collisions at the $\hat{I}\yen (4S)$ resonance. The measured branching fraction $\mathcal{B}(B^0\to J/\ddot{I}ˆ\ddot{I}‰)=\left(2.16\pm 0.30\pm 0.14\right)\times 10^{-5}$, where the first uncertainty is statistical and the second is systematic, is more precise than previous results and constitutes the first observation of the decay with a significance of $6.5$ standard deviations.